Ampeg SVT classic fault

[nevetslab]

I'd start with removing the preamp, pull the PCB out and do a thorough inspection of the solder joints on the two 2-pin headers that bring in the 6.3VAC heater supply and the +/- 3.5VDC that runs the relay. I find solder fractures on those header pins regularly. Also at the other end of the board...the 5-pin I/O power header...same thing. The IC at that end has the LED display circuitry that gets it's feed from the power amp PCB. IC2B on the power amp board is the Fault Detector IC, so that circuit controls the relay power logic, as well as sending the two status voltages that lets the relay work or not. The Power Amp PCB may not have C38 across pins 6 and 7 of U2...0.1uF, which is there to prevent false triggering of the Fault detect. Axial leaded monolythic ceramic part. I often have to add that part...tack-soldered in from the solder side of the PCB.

Also, look closely at the solder joints of the two 5-pin 0.062" square header pins along the middle front edge of the power amp board....they regularly have solder joint fracture rings. Also the two bias pots on the rear edge of the same board for the same thing. Ampeg uses those pots, along with the phone jacks and XLR connector on that rear edge to support the PCB. "Don't need no stinkin' mechanical brackets to support PCB's! Seems to be their educated engineering logic. Solder joint fractures are the most common cause of failure on those amps!

On the Power Amp PCB, the AC Heater wires that feed the Preamp heater plugs into the right-angle 2-pin header. I usually don't find solder joint failures on those terminals, but do find problems on the right-angle insulated quick-disconnect on those heavy white AC leads from the Heater Xfmr. You've no doubt encountered those in the past being crusty and overheated when oxidation sets in over time. Make sure those connections are solid. Are the 220 ohm screen resistors present, or still 22 ohms with diodes across them?

The AC Mains Relay PCB. Make sure you have solid connection from the one control wire between the Power Amp PCB and the relay. It goes out on J12 on the big board and comes in on J35 on the Relay PCB. While looking at the bottom side of that PCB, repair any solder joint fractures you find on the IEC AC Mains connector and the fuse holder next to it. Both regular candidates for solder joint fractures.

Last week, I had a client's SVT-CL that I had published details on just recently, it having the traditional solder joint fractures thruout the amp, and repaired all those. When I had reassembled the amp, let it warm up for half an hour, having gotten busy making my morning coffee and other morning tasks getting my day started, I pulled out my Fender Bass and plugged into it. Worked fine at moderate levels, so ran it up to a higher level, hit an open A-string hard, and it shut down, seeing my power analyzer display go blank briefly, came back, while the amp had gone quiet, went RED, then Protect Light began flashing. I had pulled the amp apart again and had it that way for over a day, never finding anything to account for that.

When I had moved my Valhalla 2101 Power Analyzer, I momentarily lost AC line voltage reading, looked at the rear binding post connections where AC Source and AC Load connect, and found one of the four binding posts was loose. Tightened that one and the other three. Hadn't checked those or addressed their tightness in over 10 years. Meanwhile, I had hammered hard on my bass thru the amp, still standing vertically out of the cabinet as I place them when screening power tubes, and never did get a repeat of the Protect Mode. Put it back together, after inspecting the floor of the cabinet for any foreign material that shouldn't be there close to the power amp motherboard. Played the amp a couple more hours reassembled with no further shutdown issues. So, I think that one shutdown was from the loose AC Mains connection on my power analyzer, thru which I was powering the amp. So, Solid AC Mains connection to the amp can also be a factor, is what that episode taught me.

[pontiacpete]

I pulled the pre amp board and soldered all of those connections, realizing that I had already done this last week but I thought it would be worth going over it. On the power board(I guess you mean the biggest board) I resoldered the big connector from the pre amp and the bias pots and basically most connections on the board. Then I see you mentioned the relay board, hadn’t done anything with that, so I re-soldered it all. This time it seems to be holding. I hit it hard on the bass with a good volume and hard attack. I will continue to check this tomorrow, and if all goes well I’ll put it back in the cabinet. I hate the solder they use in most of this amp. I’ve been sucking it all off and putting 60/40 in.
This amp does have C38 in place, one of those little yellow drop types, 104.
Thanks for explaining all this. I know I’ve read a lot by you on these amps and you are repeating things. Sorry, but I really appreciate the help.

[nevetslab]

I pulled the pre amp board and soldered all of those connections, realizing that I had already done this last week but I thought it would be worth going over it. On the power board(I guess you mean the biggest board) I resoldered the big connector from the pre amp and the bias pots and basically most connections on the board. Then I see you mentioned the relay board, hadn’t done anything with that, so I re-soldered it all. This time it seems to be holding. I hit it hard on the bass with a good volume and hard attack. I will continue to check this tomorrow, and if all goes well I’ll put it back in the cabinet. I hate the solder they use in most of this amp. I’ve been sucking it all off and putting 60/40 in.
This amp does have C38 in place, one of those little yellow drop types, 104.
Thanks for explaining all this. I know I’ve read a lot by you on these amps and you are repeating things. Sorry, but I really appreciate the help.

Always a pleasure to be of service, and pass along those little tidbits of grief these amps can inflict on us all. I had two of these last week, both of which had popping and crackling occurring, Lead-Free solder. One was a later generation, that has that long single-in-line replacement of the 20-conductor ribbon cable that had the dual row connector. The preamp PCB is now double-sided, and with lead-free solder, I tried to remove the Master Volume pot to clean it, and couldn't get the bloody thing out of the PCB! And, needing to attack it from the top side of the board as well to get the rest of the solder out of the plated-thru holes of those pots, there's no angle of attack to get at the connections! Miserable curr! You could envoke the problem by firm whack on the top of the cabinet. I unsoldered/resoldered the Master Volume, both tube sockets, all of the film caps in the signal path, and tried again. That cured it. The other one had the same problem, but wasn't the double-sided PCB, and still had the ribbon cable/dual row connector. I was able to get the Master Volume and Volume pots removed, pulled apart and cleaned/lubed the resistance & wiper tracks. Later generation PCB layout, so didn't have that horrible cheap PCB material whose foil pattern fails with the greatest of ease. On both amps, nothing was visible to account for the popping and crackling, but de-soldering/resoldering cured the problem. Hate Lead-Free Solder.....can't use my Pace MBT Desolering Iron on that stuff.